As is known in the art, materials are selected for use based on criteria including minimum strength requirements, useable life, and anticipated normal wear. Safety factors are typically factored into design considerations to supplement material selection in order to aid in reducing the risk of failures including catastrophic failure. Such failures may occur when the required application strengths exceed the actual material strength. During its life, the material is weakened by external events such as mechanical and/or chemical actions arising from the type of application, repeated usage, hurricanes, earthquakes, storage, transportation, and the like; thus, raising safety, operational, functionality, and serviceability issues throughout the materials life. Non-Destructive Inspection (herein after referred to as “NDI”) is carried out, at least in part, in order to verify that the material exceeds the minimum strength requirements for the application.
Since its inception in the early 1900s, the NDI industry has utilized a variety of techniques and devices with the majority based on the well known and well documented techniques of magnetic flux leakage (herein after referred to as “MFL”), magnetic particle, eddy-current, ultrasonic, radiation, such as x-ray and gamma ray, dye penetrant, and dimensional as well as visual and audible techniques. These techniques have been utilized alone or in combination with each other to address the specifics of the Material-Under-Inspection (herein after referred to as “MUI”). A list of typical MUI includes, but is not limited to, engine components, rails, rolling stoke, oil country tubular goods (herein referred to as “OCTG”), chemical plant components, pipelines, bridges, structures, frames, cranes, aircraft, sea going vessels, drilling rigs, workover rigs, vessels, structures, other components of the above, combinations of the above, and similar items.
A prolifically used MFL inspection unit is found in U.S. Pat. No. 2,685,672 and in particular, the sensors and their arrangement as described in FIGS. 5 and 6 are still in use today with hundreds of units employed worldwide in portable or stationary configurations. The same sensor configuration is also described in FIG. 7 of U.S. Pat. No. 2,881,386. This type of NDI is commonly referred to as Electro Magnetic Inspection (herein after referred to as “EMI”). These types of units utilize a magnetizing coil to induce a magnetic field into the MUI. It should be understood that the magnetic field can be applied in any direction. U.S. Pat. No. 2,685,672 shows the induction of a longitudinal magnetic field while U.S. Pat. No. 3,202,914 shows the induction of a transverse magnetic field.
Since these types of EMI units deploy a single sensor per area of MUI, they are classified as one-dimensional herein after referred to as “1D”)units. In this type of EMI unit, the signal of each 1D sensor is typically amplified and filtered by a high-pass filter for system stability and by a low-pass filter to remove the noise. Referring to FIG. 6 of U.S. Pat. No. 3,202,914, capacitor 51 and its associated components form a high-pass filter while capacitor 48 and its associated components form a low-pass filter. The highest signal is then selected for presentation to the inspector under an assumption, now widely accepted as being false, that the highest signal corresponds to the worst imperfection. EMI units with multichannel chart recorders assign sensors to a group and then they select the highest signal within the group for presentation to the inspector.
The main drawback of these EMI units is that they are one-dimensional and specifically, that the signal of any 1D sensor can be used as a variable to only one equation. Centuries of strength-of-material knowledge however, define NDI as a multidimensional problem that can never be solved in 1D. The severity of any imperfection, its failure-potential, is a function of its overall geometry, its immediate neighborhood and the loads applied to the MUI, but it is never a function of its response to magnetic excitation. In fact, large 1D signals often arise from well known material features such as braces, tapers, etc. while the detection of dangerous defects, such as fissures or cracks, is the aim of NDI. This predicament, further discussed below, is also illustrated in U.S. Pat. No. 2,527,000.
It is therefore desirable to retrofit Autonomous NDI (herein after referred to as “AutoNDI”) capabilities to the hundreds of 1D EMI units deployed worldwide. The imperfection features discussed in the AutoNDI prior application Ser. No. 10/995,692, now U.S. Pat. No. 7,155,369, are derived by the extraction matrix. The extraction matrix however, cannot solve a multidimensional problem with only one variable. This problem can be solved by compensating and then analyzing in detail the frequency spectrum of the imperfections to derive a frequency based flaw spectrum for further use by the AutoNDI.
Early on, NDI recognized that the inspection sensor signals were made-up from components of different frequencies. It was readily apparent that the lower frequency components originated from material features that were large while smaller material features gave rise to the higher frequency components. Most of the large material features were often designed into the MUI, thus these features were known in advance, or they could be observed visually while the smaller features were mostly associated with imperfections and could not readily be observed visually.
This often encountered NDI predicament is addressed in the first paragraph U.S. Pat. No. 2,527,000 and it is also shown graphically in its FIG. 3. While searching for small imperfections “ . . . to discover any irregularities caused by the presence of fissures or other discontinuities in the rail . . . ” the large MUI features that were part of the original rail design “ . . . rails are joint by angle-bars, bolts . . . which constitute in themselves irregularities . . . ” interfere with the detection of the fissures “As a result, should an internal fissure occur in the rail within the region of the angle bar or closely adjacent to the ends of the angle bar it would be impossible to distinguish such fissure from any other indication”. The oil country tubular goods (OCTG) NDI face a similar problem with pipe collars (couplings) and the detection of small imperfections on or in the vicinity of the collars, which has typically been assigned to an offline manual inspection/verification crew. For example, during a trip, when tubing or drill pipe is pulled out of a well, every single joint must be broken off the stand and laid down separately instead of standing up double or triple stands. This process creates a very expensive, labor intensive, and time consuming endeavor. Therefore, it is desired in the art, to provide a simple solution to this predicament by deploying the AutoNDI features.
Early NDI units focused on the smaller imperfections by passing the inspection sensor signal through a high-pass frequency filter and thus selecting its higher frequency components for presentation to the inspector. A high-pass filter is also known in the art as Low-Reject, Low-Cut, DC-block, or AC-Coupler. The simplest high-pass filter known to the art is made up by a capacitor followed by a resistor and can be found in patents such as U.S. Pat. No. 2,582,437 (see FIG. 1 capacitor 13 and resistor 40). Two such filters can be seen in the earlier U.S. Pat. No. 1,823,810 (see FIG. 1, amplifier 20) as well as in U.S. Pat. No. 5,671,155 (see FIG. 1, AC-couplers 6) and U.S. Pat. No. 5,943,632 (see FIG. 1, AC-couplers 6).
U.S. Pat. No. 2,770,773 encompasses many elements of the above to detect corrosion pitting and clearly states a frequency separation essential element, the frequency versus scanning speed interdependence. The high-pass filters of FIG. 7 (capacitors 66, 67 and resistors 69, 70) remove many unwanted “ . . . signal producing variables such as separation from the casing wall, wall roughness, misfit . . . ” [Column 6, Line 15]. Following the high-pass filter is a band-pass filter “. . . to pass frequencies in the band between about 3 and 20 cycles per second, as this is the characteristic frequency range of signal due to the passage of the shoe 15 across a casing corrosion pit at a transverse speed of twenty feet per minute. This frequency band related to the speed of traverse of the instrument 10 through the casing 12 will, of course, be varied to suit any other traverse speed selected” [Column 6, Line 33]. It is well known in the NDI art that the frequency range of the imperfection signal is proportionately related to the scanning speed. Thus the frequency range of the filters must be adjusted for different scanning speeds, an essential element missing from U.S. Pat. Nos. 5, 671,155 and 5,943,632. Thus, it is desired, in the art, to provide a simple compensation technique to fulfill this essential element. Such a compensation technique is further described herein below.
Yet another example of this technology can be found in U.S. Pat. No. 3,202,914. The MFL signal for different types of imperfections is shown in its FIG. 5. The specification is written in terms of time which is inversely proportional to frequency and it utilizes both frequency filtering and purpose built sensors to separately process the imperfection signals. This is found at Column 5, Line 57 “Signals from the rod wear search coils 8 and 9, being of low level and low frequency . . . ” and at Column 6, Line 37 “Thus the values of resistance 50 and Capacitor 51 are chosen so that the high frequencies of the signals from the seam pass . . . ”, a classical separation of rod wear and seam signals. For the same sensor and scanning speed, rod wear, being wider than a seam by at least an order of magnitude, gives rise to lower frequency signal than the seam. Examples of this technology are inspection units offered by OEM, Inc. San Antonio, Tex.
Another early observation of the NDI industry is the scanning speed versus signal amplitude proportional interdependence for coil sensors. U.S. Pat. No. 2,881,386 (see FIGS. 10 and 11) provides a technique for amplitude compensation for the scanning speed variations.
There are many drawbacks associated with this signal processing technology. In nature, even simple imperfections are made up of multifaceted slopes, each slope giving rise to signals of different frequency. Thus, there is no single frequency component exclusively associated with any imperfection type. U.S. Pat. No. 2,770,773 associates the frequency band of 3 Hz to 20 Hz primarily with corrosion pitting signals at the scanning speed of 20 ft/min. However, other imperfections, such as the ends of wall thickness changes could also give rise to signals with frequency components within the same frequency band. The above shortfalls, of conventional inspection equipment, explain the NDI industry's extensive use of manual verification whereby the inspection unit flags an area on the MUI and the verification crew investigates further using visual, dye penetrant, mag particle, ultrasonic, x-ray, pressure testing, and/or miscellaneous other manual techniques. Manual verification is typically performed offline, utilizes specialty inspection equipment and inspectors skilled in a multitude of inspection techniques, it is time consuming, and is therefore very expensive.
Furthermore, imperfections in nature coexist and when detected give rise to even more complex waveforms that include a multitude of frequency components including their sums and differences. This often occurs in materials that endure dynamic loading, where simple corrosion pits become stress concentrators resulting in cracks forming at the bottom of the corrosion pits. The prior art does not address this complexity.
Another drawback of the conventional inspection systems, is the scanning speed interdependence of the imperfection signal frequency spectrum, discussed in U.S. Pat. No. 2,770,773, which prohibits the use of frequency filters with fixed characteristics such as the ones used in U.S. Pat. Nos. 5,671,155 and 5,943,632.